1. Technical Field of the Invention
The present invention relates to wireless communications and, in particular, to implementing polarization diversity in mobile terminals.
2. Description of Related Art and Objects of the Invention
Mobile wireless communication is becoming increasingly important for safety, convenience, and efficiency. One prominent mobile communication option is cellular communication. Cellular phones, for instance, can be found in cars, briefcases, purses, and even pockets. Cellular phones, like most mobile communication options, rely on the transmission of electromagnetic radiation from one point to another.
In general, a cellular system is composed of many cells, each with a base station antenna for receiving transmissions. From the base station, the cellular system has interfaces for routing a call through or to the land-based, or terrestrial, telephone network, often referred to as the public switched telephone network (PSTN). The base stations form one half of the cellular system. Cell phones, called mobile stations, mobile terminals, or merely terminals, form the second half of the cellular system. In short then, electromagnetic radiation transmissions between terminals and base stations are an essential component of cellular systems, and such transmissions must be optimized by the cellular system to maximize cellular phone service, quality, and availability.
When communicating via the transmission of electromagnetic radiation, diversity can be used to counteract signal fading, which occurs when a signal's strength decreases. A given radio signal will usually take multiple diverse paths from the transmitter to the receiver. These multiple paths arise from the signal taking a direct path or any one of many reflective paths. As a result, the receiver actually has several versions of the same signal from which to choose for processing. Often, the different paths will not be fading simultaneously, so if the receiver can always be processing the version of the signal with the least fade at a given moment, then the overall transmission will be more reliably received and processed. This is termed path diversity. Diversity in general, however, can be applied to various techniques for creating and/or selecting the current optimum version of the signal.
Referring now to FIG. 1, an example of the benefits of diversity is illustrated. A Graph of Time versus Received Signal Level is provided at 100. Graph 100 represents a mobile terminal with two signal levels from which to choose for processing. Signal One is diagramed at 110, and Signal Two is diagramed at 120. If Graph 100 represents path diversity, then each signal represents the signal arriving via a different path. In Graph 100, selection diversity is implemented by selecting the strongest signal at any given instant for processing. The strongest signal is diagramed at 130 as the Diversity Signal. At Point in Time 140, for example, Graph 100 demonstrates how the mobile terminal switches from processing Signal One 110 to Signal Two 120. Diversity Signal 130 demonstrates how selecting the stronger of Signal One 110 and Signal Two 120 effectively creates a higher averaged received signal power. (It is noted that Diversity Signal 130 is diagramed above the higher of Signal One 110 or Signal Two 120 instead of directly over for diagrammatical clarity.)
In addition to taking advantage of diverse paths (path diversity as described above), the receiver can have two antennas. If the antennas are spaced sufficiently far apart (approximately 0.25.lambda. for mobiles and 10.lambda.-20.lambda. for base stations), then one antenna is often in a better reception position than the other antenna at any given instant. Using two spaced-apart antennas is termed space, or antenna, diversity. If Graph 100 represents space diversity, then each signal of Signal One 110 and Signal Two 120 represents the signal arriving on a different antenna of the two antennas. The Diversity Signal 130 represents the signal selected for processing that is currently strongest, and Point in Time 140 represents a point in which the receiver is switching from one antenna to the other.
Two other examples of diversity are frequency and time. Frequency diversity requires that the transmitter transmit the same signal over two different frequencies because when the frequencies are sufficiently far apart, their fading should vary sufficiently to allow one signal to frequently be strong when the other is fading. Time diversity requires that the transmitter transmit the same signal at two different times because when the duration between the transmissions is sufficiently long, their fading should differ sufficiently whereby the earlier or later signal will be strong while the other is fading. With respect to FIG. 1, Signal One 110 and Signal Two 120 would represent signals transmitted either on different frequencies or at different times. While both frequency and time diversity are useful techniques, they require duplicative transmissions, which waste transmitter resources, e.g., power transmitted.
Another example of diversity is polarization diversity. Polarization diversity improves the average power of the processed signal because signals transmitted on orthogonal polarizations exhibit uncorrelated fading. For example, with respect to FIG. 1, the vertically polarized electromagnetic signal may be represented by Signal One 110, and the horizontally polarized electromagnetic signal may be represented by Signal Two 120. Diversity Signal 130 then represents the selection of the better of the two orthogonally polarized signals.
Polarization diversity reception has occasionally been used at base stations. Referring now to FIG. 2, polarization diversity reception at a base station is illustrated at 200. Base station tower 210 includes a vertically polarized antenna 220 and a horizontally polarized antenna 230. For mobile terminals (which in this application encompasses all portable communication devices, including, but not limited to, cellular phones, citizen band radios, walkie-talkies, etc.), however, there is normally only one transceiving antenna. Those mobile terminals that do have a second antenna (for example, for space diversity) typically employ antennas with low gain, vertical polarization, and omni-directionality. These antennas are usually of monopole or dipole derivatives.
Mobile terminals, whether they are cellular radios, mobile radios, or other types of mobile terminals, can be positioned at any orientation. If they are portable-sized (e.g., approximately 10.times.8.times.3 inches), then they might be either laid flat or set upright. If they are hand-held-sized, then they can potentially be held at any orientation during use. Unfortunately, because prior art mobile terminal antennas have a fixed polarization (usually vertically polarized when held in hand and sitting straight up), non-optimum reception results when mobile terminals are held at an orientation other than the one for which they were designed. This becomes an even more acute, system-wide concern for critical communications to terminals such as those used by public safety and similar agencies because these terminals must function adequately even in a network's fringe areas.
As discussed immediately above, hand-held mobile terminals may be held at any orientation during a call. This causes non-optimum reception because most base stations transmit vertically polarized signals. Furthermore, the environment through which electromagnetic signals travel can cause scattering and fading. Scattering can alter the polarization of a signal's wave front before it reaches the mobile terminal's antenna. Fading exacerbates the reception difficulties because it can cause the signal with the best polarization to be the weakest. In other words, the strongest signal may not be in the orientation that the mobile terminal's antenna was designed for. A device or technique for increasing the likelihood that the strongest signal can be received by a correctly polarized antenna is needed.
In summary, mobile terminals have heretofore only incorporated at most two antennas, both of which were of the same polarization. Also, polarization diversity reception has heretofore only been used at base stations and, even then, with only two diversity branches.
A non-exhaustive list of objects of the invention follows:
An object of the invention is to provide a mobile terminal that uses polarization diversity.
An object of the invention is to provide a mobile terminal that uses polarization diversity with three branches.
Another object of the invention is to provide a mobile terminal whose polarization diversity branches are combined using a variety of techniques and algorithms.
Another object of the invention is to implement a three-branch polarization diversity receiver capable of being well-matched to any incoming transmission, regardless of polarity.
Yet another object of the invention is to improve a mobile terminal's reception with respect to polarization diversity.
Yet another object of the invention is to improve an entire communication system by implementing polarization diversity in a mobile terminal.